1. Field of the Invention
This invention relates to RF probes for nuclear magnetic resonance (NMR) spectroscopy and microscopy and, more particularly, to resonant coils for the transmission and reception of NMR signals.
2. Description of Related Art
In an NMR spectrometer probe, a sample is placed in a static magnetic field which causes atomic nuclei within the sample to align in the direction of the field. Transmit and receive coils, which may be combined in a single coil or set of coils, are placed in the probe positioned close to the sample. The transmit coils apply an RF magnetic field orthogonal to the direction of the static magnetic field, perturbing the alignment of the nuclei. The transmit signal is then turned off, and the resonant RF signal of the sample is detected by the receiver coil. The sensitivity of the spectrometer depends on a number of factors, including the strength of the static field, the closeness of the coupling between the RF coils and the sample, and the resistance of the RF coil.
Currently, most commercial NMR spectrometers use RF coils made of a normal metal, such as copper, or a combination of normal metals. Much research has been devoted to the design of coils for maximum sensitivity. For example, to achieve close coupling, coils have been made that include configurations such as solenoids, saddle coils and birdcage coils, all of which have high filling factors. In each case, however, the resistance of these coil materials has limited their sensitivity. Cooling of RF coils to reduce their resistance has been suggested. However, even when cooled, the sensitivity of conventional normal-metal coils is still limited by their resistance.
The use of superconductors in place of conventional normal metal for RF coils in NMR spectrometers has previously been suggested. For example, U.S. Pat. No. 5,247,256 to Marek describes several RF receiver coil arrangements for NMR spectrometers using thin-film superconducting coils.
The advantage to be obtained with high temperature superconductor (xe2x80x9cHTSxe2x80x9d) coils is significant. HTS coils have very low resistance and are operable in high magnetic fields at temperatures achievable with currently available refrigeration systems (above 20K). The quality factor, Q, of the coil is a useful measure of the coil""s efficiency. Q=xcfx89L/R, where xcfx89/2xcfx80 is the resonant frequency, L is the inductance and R is the resistance of the coil. Well-designed room temperature NMR coils achieve matched Qs of about 250. Because of the extremely low resistance of HTS coils, coils with matched Qs of 10,000 or more are possible. However, this advantage can only be realized if the other factors necessary for a superior NMR probe are met, such as a reasonable filling factor and high RF and DC field homogeneity.
In addition to Marek, others have reported thin-film superconductor RF coils for magnetic resonance applications. For example, U.S. Pat. No. 5,276,398 to Withers, et al. describes a thin-film HTS probe for magnetic resonance imaging. It discloses a thin-film coil having inductors in a spiral of greater than one turn and capacitive elements extending from the inductors. Withers thus provides a thin film distributed capacitance probe coil. However, magnetic field disturbances by the coil can be a problem, and the current carrying capacity of the coil is somewhat limited.
U.S. Pat. No. 5,258,710 to Black also describes HTS thin-film receiver coils for NMR microscopy. Black discloses several embodiments, including split ring, solenoidal, saddle coils, birdcage coils and coils described as xe2x80x9cHelmholtz pairs.xe2x80x9d Black""s embodiments are essentially conventional NMR coil designs and do not address the unique characteristics of high-temperature superconductor materials. Superconductors are very attractive for use in these coils. They have very low resistance at radio frequencies and, hence, produce little noise. Even so, to obtain high signal-to-noise ratio (SNR), the coils must be as close as possible to the sample. Unfortunately, this means that any magnetization of the coil material will affect the uniformity of the DC polarizing field (B0) over the sample volume, producing a distortion of the spectral line shape and degradation of SNR. Because superconductors are strongly diamagnetic, line-shape distortions could be severe.
Thin-film HTS coils offer design and processing challenges not present with normal-metal coils. First, high-temperature superconductors are perovskite ceramics, which require a well-oriented crystal structure for optimum performance. Such orientation is extremely difficult to achieve on a nonplanar substrate. Generally, such coils are preferably deposited epitaxially on a planar substrate. This makes the achievement of a high filling factor more challenging. It is also desirable for the coil to be deposited in a single layer of superconducting film, without crossovers. Second, the coil must be able to handle relatively high currents while producing a uniform magnetic field and avoiding distortion of the B0 field of the magnet. Even when HTS films are deposited epitaxially on a planar substrate, crystalline defects inevitably occur. This can lead to burn out of fine features of a coil exposed to high currents. Third, it is well known in the art that forming ohmic contacts between an HTS and a normal metal is difficult and generally leads to parasitic losses at the point of contact. To the extent that a normal metal is used in the coil, resistive losses in the metal elements would lessen the advantages gained from the use of the HTS. Thus, an ideal probe should avoid normal-metal conductors in series with the HTS.
U.S. Pat. No. 5,565,778 to Brey, et al. discloses a number of different configurations of a probe for NMR spectroscopy. Each of these configurations uses a coil having conductors mounted on a planar substrate. The conductors are arranged such that the coil includes at least one interdigital capacitor. That is, interleaved conductors having a constant spacing between them are located on the substrate. Each conductor surrounds a central sample location and lies closely adjacent to at least one other conductor. None of the conductors completely surrounds the sample location on its own, but the conductors are in an alternating arrangement such that adjacent conductors have respective breaks in their conductive paths at different radial positions relative to the sample location. This results in a capacitive configuration that forms a coil surrounding the sample location.
In accordance with the present invention, a magnetic resonance radio frequency resonator is provided that makes use of a particularly efficient coil configuration. The resonator generates a radio frequency magnetic field in an active sample volume, and includes a planar substrate on which is deposited a conductive material. In a preferred embodiment, the conductive material is superconducting, and is preferably a high-temperature superconductor. It is deposited on the dielectric substrate so as to form a plurality of nested current carrying loops, each of which has magnetic field generating elements and interdigital capacitor elements. These elements are arranged to form a substantially closed geometric path surrounding an inner region that lies adjacent to the active volume, and that is essentially oblong in shape. That is, the shape is longer along a major axis than it is along a minor axis perpendicular to the major axis. In the present invention, most or all of the capacitor elements are located in the current carrying loop farther from a center of the oblong shape than the magnetic field generating elements.
In the preferred embodiment, the magnetic field generating elements run substantially parallel to the major axis of the oblong shape. In contrast, the capacitor elements run perpendicular to the major axis. The capacitor elements comprise conducting fingers that are separated by non-conducting gaps. In one embodiment, the magnetic field generating elements vary in length relative to their distance from a center of the oblong shape. Likewise, the capacitor elements may also have different lengths relative to their distance from the center of the oblong shape. Typically, this would result from the electrical connection between individual magnetic field generating elements and their corresponding capacitor elements, which results in the outer elements being longer than the inner ones. Such an arrangement may therefore result in an overall space occupied by the magnetic field generating elements being essentially trapezoidal in shape.
The connection between the magnetic field generating elements and the capacitor elements may vary. In one embodiment, each magnetic field generating element is connected to a plurality of capacitor elements, while in another embodiment, each capacitor element is connected to a plurality of magnetic field generating elements. The total number of capacitors formed by the loops may also vary from one embodiment to another, typically depending on the necessary resonant frequency. The capacitors are arranged in series with each other and with the magnetic field generating elements, and may total, for example, two or four. One embodiment, however, uses just a single capacitor formed by the capacitor elements. A variation of the present invention that is particularly useful for a one-capacitor coil makes use of sub-coils, each of which incorporates a plurality of the magnetic field generating elements and the interdigital capacitor elements. Preferably, the capacitor elements in such an embodiment would be located to both sides of the minor axis of the coil. This provides the coil with a good degree of symmetry. The location of capacitor elements may alternate between sub-coils, such that the capacitor elements of a first sub-coil are located to the opposite side of the oblong shape from the capacitor elements of any sub-coil immediately adjacent to it. Finally, it may be desirable to use isolated conductive segments within the resonator to occupy what would be otherwise unoccupied space. These isolated segments are not electrically connected to the current carrying loops, but serve to assist in the exclusion of RF magnetic fields from the coil.